assistant: Add action footer and refine slash command popover (#16360)

- [x] Put the slash command popover on the footer
- [x] Refine the popover (change it to a picker)
- [x] Add more options dropdown on the assistant's toolbar
- [x] Add quote selection button on the footer

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Release Notes:

- N/A

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Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>

crates/assistant/src/assistant.rs

@@ -9,6 +9,7 @@ mod model_selector;
 mod prompt_library;
 mod prompts;
 mod slash_command;
+mod slash_command_picker;
 pub mod slash_command_settings;
 mod streaming_diff;
 mod terminal_inline_assistant;